Grain size of metal castings

ABSTRACT

A DEFINITE RELATIONSHIP IS DISCLOSED BETWEEN CAST STRUCTURE AND A PARAMETER WHICH IS OBTAINABLE FROM EXISTING PHASE DIAGRAM INFORMATION. THE PARAMETRIC RELATIONSHIP   WHEREIN ML IS THE LIQUIDUS LINE SLOPE, K0 IS THE SOLUTE DISTRIBUTION COEFFICIENT AND C0 IS THE BULK COMPOSITION IN TERMS OF ATOMIC PERCENT OF THE GRAIN SIZE REFINING ADDITIVE.

Sept. l2, 1972` |...A.TARsms ETAI- 3,590,874

I y GRAIN SIZE 0F METAL CASTINGS Filed Dec. 5, 1969 3 Sheets-Sheet l Lemue/ A.7rsh/S, dames L. Wei/ker',

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Sept. l2, 1972 A. TRsl-us TAL 3,690,874

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Lemue/A 7zrs/vs) (if Th efr* Attorney.

United States Patent ce 3,690,874 GRAIN SIZE F METAL CASTINGS Lemuel A. Tarshis, Latham, and James L. Walker, Schenectady, N.Y., assignors to General Electric Company Filed Dec. 5, 1969, Ser. No. 882,512 Int. Cl. C22c 19/00 U.S. Cl. 75-170 2 Claims ABSTRACT OF THE DISCLOSURE A definite relationship is disclosed between cast structure and a parameter which is obtainable from existing phase diagram information. The parametric relationship distribution coeicient and Co is the bulk composition in terms of atomic percent of the grain size refining additive.

The invention herein described was made in the course of or under a contact or subcontact thereunder with the Department of the Army This invention relates to the as-cast grain size of metals and more particularly to the provision of a method for producing a preselected reduction in the grain size of castings by the addition of small amounts of grain refining materials.

More specifically, it has been found that a definite relationship exists between the structural characteristics, namely grain size, of metal castings and a parameter which utilizes existing, well-known phase diagram information. 'Ihis permits the pre-selection of grain size in a given casting by the addition of relatively small amounts of appropriate alloying agents on the basis of readily available, published phase diagram information. In general, the process of the invention involves the concept that the nature of solute redistribution in castings controls the resulting grain size and is related in a denite manner to the bulk composition of the solute, the solute distribution coefficient and the phase diagram liquidus line slope, all of which are obtainable from known, published data, particularly with respect to binary alloy systems. In more complex systems, the major alloy constituent is usually the solute constituent controlling as-cast grain size and the binary criteria based upon the phase diagram information of the major vconstituent and the grain-size controlling additive may be applied according to the parametric relationship to be subsequently discussed in detail.

In order to more fully disclose the invention, the following detailed description of the invention will be made with reference to the accompanying drawings in which:

FIG. 1 is a graph illustrating the measured relationship of the as-cast grain size of nickel alloyed with one atomic percent of various elements to the previously mentioned parameter;

FIG. 2 is a portion of a hypothetical phase diagram of a binary system;

FIG. 3 is a graph similar to FIG. 1 in which the base metal is aluminum.

The invention may best be understood by recourse to FIG. 1 of the drawing wherein relative as-cast grain size as the ordinate is plotted against the parameter P as the abscissa for a number of alloys of nickel. With 3,690,874 Patented Sept. 12, 1972 respect to the axis of ordinates, the value 1.0 is assigned to the average as-cast grain size of pure nickel which was cast under the conditions subsequently set forth. The values less than unity represent the relationship of measured smaller grain sizes to the as-cast grain size of the pure metal as determined by the fractional expression R/Ro wherein Ra is the average measured grain size of a dilute alloy having a relatively smaller grain size and Ro is the measured grain size of the pure metal. Thus, for example, the average measured grain size of the pure cast nickel, R0, was about 3.7 mm. and that of the alloy nickel plus 1 atomic percent chromium was about 1.88 mm. The fraction 18%; of these grain sizes equals the relative grain size 0.508 or about 0.51, as plotted. With respect to the abscissa parameter expression mL(1"Ko) Co K0 m is the slope of the binary phase diagram liquidus line, K0 is the solute distribution coeliicient, and Co is the bulk composition. The solute distribution coeicient, K0, is the ratio of the solute composition of the solid to the solute composition of the liquid at an appropriate temperature. These values are determinable from a typical binary equilibrium phase diagram as follows. A portion of a hypothetical binary phase diagram is shown in FIG. 2 wherein, as conventional, the plot includes temperature in degrees Celsius as the axis of ordinates and the composition in atomic percent of the constituents as the axis of abscissas. Line L is the liquidus line and line S is the solidus line, e.g., for any given composition, all material at temperatures above L is liquid, all material below S is solid, all material at temperatures between L and S is a mixture of liquid and solid materials and TB is the melting point of the pure base metal. Using such a binary diagram, the values of the parameter are determined as follows. Assume the parameter values are to be determined for an alloy having the composition X atomic percent alloy addition and 10G-X atomic percent base metal, shown by line C. In terms of the parameter P, X atomic percent is equal to Co. The parametric slop m is that of a straight line drawn tangentially to the liquidus line at the intersection of lines C and L, as shown. The composition of the solute in the solid and the liquid of the assumed alloy is found at the points Cs and C| respectively, in terms of atomic percent of the alloy addition at the temperature TA. At the temperature TA, the ratio of Qa Cl.

equals the solute distribution coefficient Ko of the parameter P. When the composition of the alloy involves very dilute solutions, i.e. where Co is of the order of 1.0 or less, satisfactory approximations of parameter values may be made by assuming linear relationships for lines L and S and for slope mI For example, the slope m| may be taken as the slope of the liquidus line L at its point or origin as shown by line m| and an arbitrary temperature TA which is less than TB is selected. When such an approximation is made, the closer TA is to TB, the more accurate the approximation will be. It will be appreciated, however, that the degree of accuracy attainable by this technique will depend upon the accuracy of the phase diagram plot and the degree of linearity of those portions of the liquidus and solidus lines under consideration. In this connection, it will be noted that certain binary systems exhibit almost perfect linearity in these zones, for example, the phase diagram for the aluminum-copper system. Furthermore, an extremely high degree of compositional accuracy is not needed, nor, in fact, is it obtainable in conventional, useful melting and alloying practice.

The curve illustrated in FIG. 1 was determined in the following manner. Two pound castings composed of substantially pure (about 99.99) nickel and nickel containing about 1 atom percent cobalt, aluminum, chromium, copper, silicon, germanium indium, tin, lead, bismuth and cerium were prepared as follows. Substantially pure nickel was melted in a clean ceramic crucible under a positive pressure of argon in a closed system. The argon was pumped out while the nickel was still molten and the melt permitted to solidify under a vacuum. The nickel was then remelted under argon and where appropriate, the alloy additions made. The various melts were each then poured with a 50 C. superheat into a two inch diameter by four inch copper mold and solidified therein under identical conditions. The castings were cut axially and examined metallographically. The as-cast average grain sizes of each of the several specimens were determined by the straight line intercept counting technique and the corresponding relative grain sizes determined, and the parametric factors and the corresponding values for P were determined as follows:

TABLE I Relative grain Alloy size mL KD p N 1.0 N1 plus Cr. 0.508 0.5 0.25 1.5 Nl plus Bl.-... 0.054 8.5 (l) Nl plus Ce- 0.080 11.0 0 (l) N1 plus Pb...-. D. 095 16. 5 0. 08 100 Nl plus Al.. 0. 675 2.1 0. 7 0. 9 Nl plus In. 0. 149 14. 7 0.5 15 N1 plus Si.. 0. 271 6. 3 0.5 6. 3 Nl plus Sn. 0. 124 12. 5 0. 27 33. 8 Ni plus Ge.. 0. 175 2. 1 0. 15 12 Nl plus Co... 0.760 -l- .43 1 (2) Nl plus Cu 0.433 1.7 0.4 2.5

1 Much greater than 100. l Less than 1.

Plotting the relative grain size values versus the parameter values from the foregoing table produced the curve shown by FIG. 1, the values for m and Ko being determined from published binary phase diagrams.

A similar set of castings were made using high purity (about 99.993 percent pure) aluminum and such aluminum alloyed with about one atomic percent each of lithium, zinc, silver, magnesium, copper, silicon, calcium, cerium, indium, nickel, germanium and tin. The pure aluminum and these aluminum-base alloys were melted in vacuum in high-purity alumina crucibles and the alloying and pouring operations were performed under a positive pressure of argon except in the case of the silver alloy wherein the silver was added in vacuo. The same kind of mold was used as in the nickel-base alloys, namely two inch diameter by four inch copper. Again, the castings were sectioned and the average as-cast grain sizes determined in the same manner as for the nickel-base alloys. The values of P for each of these alloys were determined by determining the values of the parameter from published phase diagrams and are listed as follows.

*Greater than 100.

As before, the measured grain sizes of the several castings were plotted versus the parametric values P for each as shown in FIG. 3. It should be noted that the grain sizes for the cerium, indium, nickel, and tin alloys were very small and expressed as relative grain size of the order of 0.05 and hence not plotted. The value of Ko for the germanium alloy is open to question because of the uncertainty of the solidus line in the binary phase diagram and therefore has not been plotted. The grain size of this alloy was also very small.

Similar grain size determinations were made upon castings of substantially pure nickel containing ve atomic percent of chromium, silicon and cerium. The value of the parameter Ifor each of these alloys is of course five times that for the corresponding one atomic percent alloys. 'I'he relative grain size values for these alloys closely approximated the values predicted from the curve of FIG. 1.

The parametric relationship to as-cast grain size may be illustrated as follows. Assume, under a given set of casting conditions, a nickel casting has an average grain size R., and it is desired to reduce the grain size thereof to about 30 percent of that value using tin as the alloying constituent. From the curve of FIG. 1, the value of P corresponding to a relative grain size of 0.3 is about 6. From the known values of mL and Ko for the binary alloy phase diagram, the parametric relationship is =4 atomic percent Cr to be added.

it will be apparent that the same sort of application of the parametric relationship may be used for other base metals and alloys. For example, by using a commercial nickel-base superalloy having the nominal composition of 61.55% Ni, 9.6% Cr, 4.2% Ti, 5.5% A1, 15.0% Co, 3.0% Mo, 1.0% V, 0.15% B, 0.07% Zr and 0.18% C, all by weight, and adding 1 atomic percent cerium calculated as though the alloy was pure nickel, a predicted reduction in as-cast grain size was achieved compared to a similar casing in which the cerium was omitted.

From all the foregoing, it will be apparent that by using the published phase diagram information and the disclosed relationship between the parametric relationship P and relative grain size that controlled as-cast grain size in castings may be achieved. It will also be apparent that the algebraic sign of the slope m| is significant and that the value for Pin each case is a positive real number.

What We claim as new and desire to secure by Letters -Patent of the United States is:

1. A metal casting composed of nickel and tin, said casting being characterized by an as-cast grain size which is smaller than that of a substantially identical casting 5 6 from which tin is omitted, wherein the ratio of the refined References Cited grain size to as-cast unreiined grain size is 0.3, and the UNITED STATES PATENTS tin content of the casting is 0.177 atomic percent. l 2. A metal casting composed of nickel and chromium, gm; et al' said casting being characterized by an as-cast grain size 5 37008855 11/1961 swensol; 75 171 which is smaller than that of a substantially identical casting from which chromium is omitted, wherein the ratio of RICHARD O, DEAN, Primary Examiner refined grain size to as-cast grain size is 0.3, and the chromium content of the casting is approximately 4 atomic lo U.S. Cl. X.R.

Percent' 75k-138, 139, 146, 147, 148, 171; 148--32 

